The present application is based an and claims priority under 35 U.S.C. xc2xa7 119 with respect to Japanese Patent application No. 2000-194795 filed on Jun. 28, 2000, the entire content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a flat plate member with a gear portion, and a process for making the same.
2. Description of the Background
A flat plate member with a gear portion is used, for example, as a driven gear of an arm type window regulator device for a vehicle, which is driven by a drive gear driven by a motor. In general, the driven gear of the arm type window regulator requires high surface hardness in order to maintain enough resistance against wear, and so the surface hardness thereof is Vickers hardness Hv 400 to 500. This driven gear is generally manufactured by the following steps. First, a pressing process such as punching or blanking is performed on a flat steel raw material, and thereby a flat plate member with a gear portion is formed. Then, carburisation quenching and tempering are performed on the entire flat plate member.
According to another method, the driven gear having high surface hardness can be manufactured using only a pressing process. In this method, the pressing process is performed on a steel raw material made of high tension steel, and thereby a flat plate member with a gear portion is formed.
In case carburisation quenching and tempering are performed on the entire driven gear plate after the driven gear plate is formed, however, since it takes long time for heat treatment, the manufacturing cost increases. Further, since the heat treatment is performed on the entire driven gear plate, and the content of carbon whose strain is large due to the heat treatment is high, a large strain is generated on the driven gear due to the heat treatment. As a result, the accuracy of the tooth form of the gear portion deteriorates and the noise generated during the operation of the arm type window regulator device increases.
Further, if the driven gear is formed by performing the pressing process on a steel raw material made of high tension steel, the accuracy of the tooth form of the gear portion does not deteriorate and the noise generated during the operation of the arm type window regulator device is reduced. However, since the raw material made of high tension steel is expensive, the manufacturing cost is increased.
It is, therefore, an object of the present invention to provide an improved flat plate member with a gear portion, in which the accuracy of the tooth form of the gear portion after quenching can be increased without increasing the manufacturing cost.
In order to achieve this objective, according to a feature of the invention there is provided a flat plate member with a gear portion, comprising a flat steel material having a composition comprising, by weight, 0.10-0.18% C, less than 0.03% Si, 0.60-1.50% Mn, less than 0.020% P, less than 0.013% S, 0.001-0.004% B and the balance Fe with inevitable impurities, wherein only the gear portion is quenched.
According to another feature of the invention there is provided a process for forming a flat plate member with a gear portion, comprising the steps of press forming a flat steel raw material having a composition comprising, by weight, 0.10-0.18% C, less than 0.03% Si, 0.60-1.50% Mn, less than 0.020% P, less than 0.013% S, 0.001-0.004% B and the balance Fe with inevitable impurities, to a predetermined shape having a gear portion; and quenching only the gear portion.